Never Leave the Island
by booksindalibrary
Summary: Tsuna, as a merman, was supposed to stay away from humans. [Cross-posted from AO3 under the same title.]
1. Chapter 1

Tsuna was the only male to be born into the tribe of sirens – hell, the only man, boy, whatever, to be born _ever._ A merman, they called him, lacking a term for it. The women were sirens, the men were...mermen?

Sirens, the kind that lured sailors to their death. They sat on rocks, combed their hair, and sang. Tsuna couldn't sing, not like them; he could barely stand to hear himself. He had been told he was fine, he could sing – Sirens were also excellent liars.

"Tsuna," one called, gesturing. "Go help the men." That was his job, to help the stranded sailors get to the island without drowning. Tsuna obeyed, diving into the water with an agility none of the Sirens had. He couldn't sing, but he could _swim._

And the Sirens sang their prophecies, while Tsuna dragged men to their death, his four-year-old frame struggling to bear their weight.

~o0o~

"Watch out for a man named Reborn," one of the older women, about a hundred years old, told him. She only appeared twenty, but the way she held herself spoke of eons. Her eyes were pale, rather than the usual full inky black the rest had.

"Reborn?" Tsuna frowned. "Do you mean another man will be born here?"

She shook her head violently. "No. A _human._ They will learn of here, and drag you to their world." She gripped his shoulders. "The prophecy, what I saw – you will become human, and you will-" She choked on her words, then focused again. "Find it. Use the Void."

The Void was terrifying, Tsuna wanted to argue. He hated the sense of loss that came with leaving his body. He wanted to be able to see the future, like the others, but instead he had been given _that._

"Find what?"

"...Find-" And she stopped, mouthing something. "I can not tell you."

"...Elder?" He looked uncertain. "Why can't you?"

Pale eyes turned on him. "It's forbidden. Child, you will _lead_ but that is all I can say."

"Confusing," Tsuna muttered.

"Yeah. Life's like that. And remember, as I keep telling you-"

"Don't leave the vicinity of the island," Tsuna chanted.

"Exactly."

~o0o~

Tsuna allowed his mind to float, drifting almost aimlessly across the ocean. It was difficult to stay near their island, and even more so to stop himself from going all the way to the stars. But he had been told never to leave the air, and never to go near land. Tsuna obeyed, too scared to ever leave.

"Tsuna," one of the others said, and Tsuna reluctantly reentered his body.

"Yeah," he said groggily, tail flapping.

"The elder called for you."

Tsuna blinked. "Why?"

She shrugged, leaning back against the rocks. "See if I care."

Tsuna hesitated, then shoved himself off the rocks and back into the water. He pushed himself down, twisting around slightly. He swam idly, ignoring the sense of urgency that wriggled in the back of his mind.

"...Tsuna," a voice said. He stopped, tilting his head. It wasn't...a female voice.

"Yes?" He said by reflex.

"...Come...here..."

What was that voice? Where was it coming from? But he had never met anyone else off of their island, and had never, ever been called by name by a _man._

Curious, he followed the sound, eager to find the source. Whoever it was kept calling out for him, and Tsuna kept swimming.

He didn't notice how far from the island he was until he burst through the surface, gasping as he readjusted to his nose and not his gills.

"Where are you?" He called.

"Here," the voice said, and Tsuna stared upwards, awestruck.

"You're...a man?"

"Yes." The man in question crouched down, smiling. "How are you?"

Tsuna was taken aback, then glanced around. "Where am I?" There were rocks, and a cliff out beyond the sand, but no obvious sign of habitation.

"Japan."

Tsuna blinked. "Um, where is that?"

The man blinked back. "Your eyes are black. That's strange."

Tsuna touched an eyelid. "Where am I?" He repeated, more forcefully. "And, human-" He jerked back as the man reached out. "Don't touch me. I don't want your filth."

"Filth?"

"Humans are filthy. That was what I was taught."

The man paled, then burst out laughing. "Oh, gee, the Ninth was right."

Tsuna glared. "About what?"

"How arrogant you are."

"I'm not-"

The man straightened. "My name is Iemitsu," he said courteously. "How do you do."

Tsuna turned away, intending to leave. "My mystery has been solved-"

"Ours hasn't," and that was when the net closed in on him.

~o0o~

He was thrashing wildly, the sensation of _burning_ coating his tail. "Wa...ter," he gasped, immediately thankful when liquid was trickled into his mouth.

"Difficult, huh?" Iemitsu asked.

A man with a staff leaned over him. "Just like Talbot said. As soon as we catch wind of a merman, we'll find our answer."

"The next heir?"

"...Yes. He's the spitting image of the first-"

"Let me go, you bastards," Tsuna spat out, then hissed curses at them in Siren.

"-If not for the eyes," the man with the staff – or stick, whatever – finished. "But I can fix that."

Iemitsu nodded. "Do it."

The man lifted a single finger, an orange flame igniting. Tsuna froze, then cried out, "No, don't do it, humans are _disgusting-_ "

Iemitsu pinned his arms as the man pressed his finger against his forehead. Tsuna passed out as the Sky Flames overrode his mind.

~o0o~

His legs felt unnatural, his head heavy, and when he looked in the mirror, his eyes were _wrong._ But he had felt that way for years now, with no answers. His father avoided looking at him, and he didn't know why.

Unbeknowst to Tsuna, the only thing that remained from his other life was his ability to escape to the Void, an ability he rarely used, until a certain hitman named Reborn came knocking.

* * *

 **/sigh/ another fic. lmao**

 **also: prompts for this:**

 **Tsuna is a merman**

 **Reasonably describe why/how he in a merman, what that has led to and manouver him to the same start point as canon then explain how he evades discovery**

 **Tsuna astral projects into the void regularly**

 **(courtesy of torashii -)**

 **also: reviews make me write quicker...ahaha yes this is v shameless**


	2. Chapter 2

**To answer Exira's question: for this fic, escaping into the void is basically just your mind/soul leaving your body. The void is...a bit like space, I guess, but it's open for interpretation.**

 **(oh btw I had this chapter up my sleeve, I didn't write it in this time - I have too many other wips over on AO3 lmao /sob)**

* * *

Reborn knew something was up the minute he stepped into Namimori. He was hard put to pinpoint it, and the feeling didn't change even when Nana opened the door and let him in.

It didn't change at all until he met the boy called 'Tsuna'.

There was nothing surprising about him, nothing that struck Reborn as being special. And yet the cold feeling down his spine told him otherwise.

Tsuna practically fell down the stairs, something that was decidedly un-mafia-like, then laughed in his face when Reborn told him he was here to tutor him.

This annoyed Reborn like nothing else. Of all the things Tsuna could have done, it had to be to mock his age. So, being Reborn, he kicked him.

~o0o~

Tsuna was normal, as far as Reborn could tell. (He had to keep up with the lie.)

"Dame-Tsuna is nothing extraordinary," he told Leon hopefully. He was lying to himself, he knew that, but he had to convince himself otherwise. If he didn't, that itching in his trigger finger might kill him (or Tsuna). And pissing off the boss of the largest mafia family in the world would not suit Reborn. Not at all.

Gokudera arrived and tried to kill Tsuna...And Reborn was definitely expecting this. He was known for his short fuse – no pun intended – and Reborn had fed him bullshit on how he would be Decimo if he succeeded in killing Tsuna.

But Reborn, no matter what he thought, couldn't let the boy die here. So of course he used the Dying Will Bullet, and continued to in every situation Tsuna stumbled into.

When he fired off his Dying Will Bullet, each and every time, he knew what to expect. All of this was normal. There was absolutely and undeniably _nothing special_ about this boy. (It was a mantra he had to repeat, over and over like a melody.)

The uneasiness never left Reborn. When he received word of Mukuro, it only increased dramatically, gnawing on his insides. It coiled around his gut, and Reborn had to wonder if his initial feeling of unease was correct. He had always trusted himself before, and now was the time to trust himself again.

Reborn was the best damn thing to walk the planet and he was. Not. Wrong. About _anything._

Tsuna, in true _dame_ fashion, spent most of the time on Kokuyo land panicking or worrying over the others' injuries. While it struck Reborn as false, he didn't see through it properly until Reborn fired off the new bullet Leon produced.

And Tsuna changed. When he stood his eyes-

- _oh god his eyes_ -

And the Flame burning on his forehead twisted as though _screaming_ , his pure black eyes absorbing the light. Even Mukuro took a step back in shock. Hell didn't have...whatever this was.

"Human," Tsuna said, voice empty, "I will silence you."

Reborn was sick with dread. Was Tsuna even human? But he couldn't interfere, Nono had told him not to. The illusions seemed to slide off of Tsuna, Mukuro growing increasingly panicked. Reborn furtively checked to see if Gokudera or Bianchi were conscious. Neither of them needed to see this.

"What..." Reborn turned at the voice, spotting Hibari propping himself up, eyes fixed on Tsuna. "The herbivore is not really a herbivore, after all."

Reborn watched the fight (which was admittedly one-sided), saying, "Tsuna probably isn't human."

"Hn." Hibari didn't look pleased, maybe because a decidedly un-human Tsuna could endanger Namimori.

Tsuna pinned Mukuro on the ground, shaking him violently, the illusionist dazed and confused. "Give it back," Tsuna hissed, almost pleadingly. "Human, give it back to me, I want it back-"

"Give what back?" Reborn cut in. The monster ( _he is normal_ the voice in his head screamed_ turned to the Arcobaleno, lips drawn back.

"Give it back," he said again, drooping this time. "Give me back my tail-"

He toppled as the Flame on his forehead went out, eyes melting to their normal brown (with the whites showing, Reborn realised with relief).

Vindice appeared, but Tsuna could do nothing as his muscles fought against his body. He was helpless as Mukuro was dragged away.

" _Siren, return to your land._ "

~o0o~

The parting words from Vindice haunted Reborn, even though Tsuna appeared to have no memory of those words or of his time in Hyper Dying Will.

Reborn, upon discovering this, hid what had happened from Tsuna. He had to find out what Iemitsu's plan was – did he know his son was...inhuman?

And _tail?_ Reborn had to imagine something goofy like a dog's tail, but then again, it would have to have some significance if it led to the desperation he witnessed from Tsuna.

None of this mattered, and Reborn knew it. Tsuna was a better candidate than _him._

(Or at least, he matched their ideals better than _he_ ever could.)

~o0o~

Xanxus's arrival struck Reborn over the head like a mallet, but like any good hitman, he sidestepped just in time and played it cool.

"Run," he told Tsuna as Xanxus stepped up to the mark, seemingly to blow them all away with his Wrath.

Tsuna blinked up at Xanxus. "I-I saw you," he stuttered, and Reborn had to gape.

Xanxus growled and spat out, "We've never met, trash-"

"Boss, you know him?" And Xanxus spun around and forced Varia to _shut the fuck up_ with only his gaze. When he turned back to look at his 'rival', he sneered, "I'm _definitely_ going to kill you now-"

And Iemitsu, a man that even Reborn curled his lip to, intruded to declare this a contest.

A game, Reborn wanted to chime in, for Vongola's enjoyment. Pit this brutal assassin against an innocent civilian, place bets, and mock them.

Tsuna isn't an innocent, and he might not be a civilian, an evil voice whispered in his ear. Tsuna is-

Reborn stifled it, shot to pieces, then threw it away. Like hell he was listening to that shit.

And Tsuna mulled over the man known as _Xanxus_ throughout his training – possibly to try to remember whatever he saw.

Reborn wanted to know whether this was a natural, _normal_ meeting, or something else.

The black in his eyes did not appear again, even in training or in battle. Reborn could not explain this.

~o0o~

Tsuna appeared to remember in his fight, when he said he knew Xanxus had been frozen before, for _eight years._

He knew, Reborn noted. Not he thought, he _knew._

"I saw it," Tsuna said blankly, and _black_ leaked into his eyes. There it was. The Flames warped and twisted, and Xanxus flinched from... _this._

"I saw it all, and what you said, but you still think you can win?"

"Voi, what is he talking about?" Squalo snapped, and Reborn had to tune him out, a feat for which he should received a medal for.

"You can't, because even if I die, you lose," Tsuna informed Xanxus.

And Xanxus bared his teeth in something that looked like hatred and resignation mingled together, saying, "I know, trash, but I have to fuckin' try, don't I?"

Tsuna closed his eyes briefly, and when he opened then, a pure black gleamed at Xanxus. "Very well, human. I can accept that offer."

Naturally, Xanxus lost.

Even after the efforts of his subordinates, after being frozen (again) and released by that strangely loyal prince, he _lost._

"Human," Tsuna said monotonously. "It was a good effort."

"Trash, shut _up._ "

"I will leave, but first you must tell me where it is."

"...where what is?" Xanxus still looked stunned after Squalo had blurted out his secrets. Tsuna was probably taking advantage, and if Reborn had been anyone else, he would have given Tsuna a gold star.

"My tail. Where is it?"

"What the fuck."

Tsuna only grew more agitated. "Human, look at my eyes, do you think these are like your kind's? My _tail,_ I must return to sea, or else the stench of _humans_ and I will not return, I want to go home-"

"...Trash, are you a fuckin' mermaid?"

Tsuna blinked, nearly falling over with shock. "Yes, I am," he said after a moment.

Xanxus burst out laughing.

* * *

 **Please leave feedback on your way out!**


	3. Chapter 3

Xanxus, for all his faults, agreed to help Tsuna with his tail problem. "Why not."

Needless to say, everyone was surprised – Xanxus didn't mention his sneaking suspicion on the cause of this. Regardless, Tsuna, eyes still black, asked, "What do you know?"

His gaze spooked Xanxus, who tried his best to hide it. He was sitting up now, scars receding slowly, clothes still steaming slightly from the heat. "I don't know anything."

A hiss of air. "Human, answer me. Don't you realise? Once this problem is solved, you'll have Vongola and I will have peace."

Xanxus gritted his teeth. "I said, trash-"

"Answer."

Reborn, now at Tsuna's side, wondered if he should intervene. The Cervello were fading into the background

"You look like the First," Xanxus said grudgingly. "Exactly like the portraits in Vongola Mansion."

Tsuna was still for a moment, then relaxed. "That...explains it."

Xanxus raised an eyebrow, but it was clear Tsuna wouldn't continue. "What do you plan?" Xanxus asked after a pause.

Tsuna looked at him. "Take me out to sea."

"You can do that without me, trash." Xanxus didn't bother to hide his annoyance. He ignored the murmurs of Varia.

"They all refuse," Tsuna said sharply. "Vongola works against me. You humans trap me here."

Xanxus smirked. "Fine. Varia can sort that."

And so, a tentative alliance was formed.

~o0o~

Iemitsu, oddly enough, did nothing to stop them. On orders from the Ninth, ones no one else knew of.

"Are you sure?" He had asked, doubting Timoteo's words.

"Yes," The Ninth replied, and that had been the last exchange before Xanxus had been unfrozen.

Allow Tsuna to do as he wants, Iemitsu thought, disgusted. They wanted to keep the boy there, not let him go out to sea.

They knew nothing of the Sirens' magic, had got lucky with Tsuna's human form. It was a miracle none of the Sirens had come for their heads, either.

Iemitsu, despite his doubts, allowed them all to board the Varia boat. Reborn as well, Iemitsu noted, worry gnawing on his innards. Hopefully they were taken care of by the monsters that lurked under the waves.

~o0o~

Xanxus studied the boy who stood on the prow. His gaze hadn't left the horizon since they had taken off. Gokudera and Yamamoto were rattling around nervously, Reborn had his eyes shadowed with his fedora, seemingly asleep. Lower level Varia members were manning the ship; the rest of the Varia officers were forbidden from going with them. Xanxus didn't want to deal with too many annoying people.

Tsuna's eyes were, thankfully, their normal brown. Gokudera, biting on a knuckle, finally hissed, "What if Juudaime jumps overboard?"

"We'll save him then," Yamamoto said confidently.

Xanxus rolled his eyes. If Tsuna fell in, then maybe he'd get his tail back.

"We're near," The would-be Decimo turned suddenly. Xanxus gestured at a crew member to slow.

They all studied their surroundings. "Trash-"

Tsuna glared, then promptly collapsed. Gokudera caught him, yelling something. Tsuna's eyes were wide open and eerily black. A moment passed, before Tsuna returned to life, jerking out of Gokudera's grasp. His eyes retained their black as he pointed. "That way."

"How do you know?"

"I saw it," Tsuna said blankly at Xanxus.

"What you just did..." Xanxus trailed off, before returning. "That's not normal."

"My mind can leave my body," Tsuna said, still blank. "I've always been able to do it."

"Cool," Yamamoto said, grinning. Gokudera breathed a sigh of relief.

"What the fuck?" Xanxus muttered as the boat headed in the direction Tsuna pointed in.

"You'll be Decimo if this works out," Reborn said suddenly and quietly, looking at Xanxus. The Varia boss looked back at Reborn almost bitterly.

"By default."

"Even so." Reborn glanced at the other three, who were discussing something within themselves. "You have a theory."

"Yes," Xanxus said. He couldn't lie to Reborn; the hitman could kill him without lifting a finger.

"Tell me."

Xanxus rubbed at a scar. "You mentioned a seal."

Reborn's eyes were unreadable as he waited.

"What if that caused all of this? His lost tail."

Reborn smirked, but with a sad tinge. "I think it goes deeper than that."

Xanxus shrugged. "I'm waiting for the trash to get to where he wants to go-"

And the boat came to a halt, the crew looking confused. "Uh, boss-"

"What is it?" Xanxus jerked around at the sound of...songs. Mesmerized, he leaned forward, over the railing, peering down into the water. A woman was smiling up at him. Xanxus paused, sensing something wasn't right.

But the song called to him. Yes, he wanted to be Decimo; he wanted it all. And yet his hands would not come from the rail-

"It's been a while," Tsuna said gently. When had he moved to Xanxus's side? Where was Reborn-

"Xanxus, step away from the rail," a baby's voice said, but the world was blurring. What was he meant to do again?

A hand on his back, and all of a sudden the water was around him, a woman clasping his face. The last thing Xanxus knew was the cold.

* * *

 **In case you can't tell, I love Xanxus sm...ahaha I'm tragic**

 **plz leave feedback on your way out! this chapter is a bit rushed bc I got so many nice words from y'all and I just. churned this out in a couple of days ahaha /sweats/**


	4. Chapter 4

When Xanxus woke, he was greeted by gentle voices and warm rock beneath him. He tensed, opening his eyes slowly, taking in the rock beneath his body and the smell of salt on the breeze.

"-glad the island is unchanged, Elder," Tsuna was saying. Xanxus squinted at the blurry mass that was Tsuna, each breath rattling in his chest.

Black eyes and a tail, Xanxus noted. Tsuna was sitting on a rock, conversing with a young woman. Xanxus shifted his weight, struggling to get up. Both turned to stare at him as his hand slipped, Xanxus slamming back into the rock, winding himself. He gasped out a curse, drawing breath back into his body.

"Human," the woman said.

"Xanxus," Tsuna said, looking almost idle. "You're alive."

Xanxus resisted the urge to jump in the air, stamping down on stray emotions. "Trash," Xanxus said,, voice rasping in his throat. He coughed, sickened with the taste of salt water in his mouth. "What the fuck-"

"You were intended to be a meal," Tsuna informed him. "The Elder-" He gestured at the woman, "-has decided otherwise."

Xanxus fell back, eyes fixing on the woman. The 'Elder' looked too young and too attractive. Xanxus hated her on sight.

The Elder's black, empty eyes turned to Xanxus. "We may not release you. Mayhap we'll keep you, you seem amusing enough."

"Amusing-?" Xanxus spat the word out like venom. "You bitch-"

Cold eyes on him. "Shut your mouth," Tsuna said warningly. "You will not speak to the Elder like that."

Xanxus hissed out air, rage rearing in his stomach. "Shut it, you brat, I'll say what I want."

Tsuna flared up, tail slapping against the rocks loudly. Xanxus could have sworn he swore Flames flickering in his eyes.

"Why did those humans take away Tsuna from us?" The Elder broke in calmly.

Xanxus glowered at her. "I don't know. I didn't even know he wasn't _human_ until the ring battle."

"But you have a guess. Tell me."

"...He looks like the first boss of Vongola," Xanxus said grudgingly. The brat's probably told her that, Xanxus thought, half-bitter. He didn't want to be here – he didn't even remember how he got here.

...He had plunged into the ocean, he recalled suddenly. That made sense – and the face of a woman smiling at him as he sank beneath the waves. He definitely didn't like any of this. It was bad enough he had to deal with Varia.

"Assassin," the Elder said softly. Her voice irritated Xanxus, and he itched for his firearms. Resisting the urge to reach for them, he glared at the Elder and waited for her to speak.

"Do you intend on taking the boy back?" The way she said 'boy' made Xanxus think she didn't care all that much for Tsuna.

Xanxus scowled. "No. I want to be the Vongola boss, so why would I? I bring him back, and the old men would sideline me again."

Tsuna smiled grimly at Xanxus, nodding.

Xanxus clicked his tongue at Tsuna, trying to hide his uneasiness. So he had three forms, Xanxus reflected; one in which he appeared human, one with the ink eyes, and one where he was a fully fledged merman. Xanxus wondered if it'd hurt to switch-

"It hurts to switch between forms," Tsuna said as though reading his mind. (Which was in the realms of possibility, Xanxus realised with a jolt.) "Even if it's just my eyes, which is why I only did so in Hyper Dying Will Mode. Reborn's bullets..." He trailed off, looking troubled. "In any case, it'd be best for me to remain a merman; taking on a human form is unnatural."

"So you're giving me the position of Decimo?"

Tsuna smiled again, so coldly Xanxus seriously considered offering him a place in Varia. That smile was one of a _killer._

"Not quite. The Elder hasn't yet said if you'd live."

The Elder shut her eyes briefly. When she opened them, the pure blackness within them was more than just cosmetic; Xanxus was starting to consider them a reflection of their souls.

"Xanxus of Italy," the Elder said, a small smile gracing her lips, "would you like to meet the rest of the clan?"

The ocean around them turned still - as though they weren't outside. The waves that lapped against the rocks stopped, the world turning silent. Feeling his blood run cold, Xanxus looked down at the surface, seeing a woman's face staring up at him, hair swirling around her features.

The mermaid surged upwards, Xanxus jerking away from her. Her head broke through the surface of the water. She smiled at him sweetly, revealing teeth sharpened to a point.

"If you'd like," the Elder said idly, "you could stay here, as our servant."

"Are you fucking with me, trash?" Xanxus spat out. "I'm not anyone's servant."

"How about our meal?" The Elder asked.

"Do you want to be charcoal?" Xanxus hissed back, the familiar rage encasing him and igniting in his palm. The Elder stared at it, surprised, then sighed.

"I see. So that's how it is."

Xanxus froze, confused. It must've shown on his face because the Elder waved a hand down, and the mermaid Xanxus had been watching from the corner of his eye disappeared underwater again. The ocean became less glass-like, the waves reforming and lapping at the edge of the rocks again.

Xanxus breathed out, relieved. Did mermaids have control over weather?

The Elder tilted her head, studying him. "I have not been acquainted with the First Boss of Vongola, nor have I heard legends of him. However, you, assassin, remind me of a man I met."

Xanxus eyed her warily, guessing where this was going. The Second Boss of Vongola-

"I had not thought of that human for a long time," she mused. "Only a human could be so furious."

Well, that didn't go as expected. "What does that mean?" Xanxus muttered.

"We merfolk do not depend on emotion as you humans do. We live for much longer and have fewer worries."

That didn't really make sense, but Xanxus wasn't going to argue with a person who commanded a horde of mermaids ready to devour him, who could seduce him with songs, and had potential mind-reading ability _and_ power over the weather.

...Yeah, Xanxus wanted to return to the mafia world and be normal.


End file.
